Beam Intensity and Penetrability Inverse square pt1

Quantity

Refers to the intensity or total number of x-ray photons in the beam.

Quality

Refers to the penetrability or energy of the x-ray photons.

Intensity

Measured in Air Kerma (Joules per kilogram). 1 J/kg = 1 Gray (Gy). Output is expressed as radiation exposure per unit of time at a given location (Gy/min).

Output

Exposure rate = Exposure (Gy) ÷ Time (min). Total exposure = Output × Exposure time.

5 Factors That Influence Output

1. Tube Current (mA)

2. Exposure Time (s)

3. Tube Potential (kVp)

4. Distance (SID)

5. Filtration (mm Al equivalent).

Tube Current (mA)

Increasing filament current creates more electrons, resulting in more electrons striking the target. Output is directly proportional to mAs. A change in mAs affects quantity but does not affect the quality (penetration) of the radiation.

Tube Potential (kVp)

A rise in kilovoltage increases output because electrons are accelerated faster, producing more photons at the target. The photons also have higher energy. Increasing kVp increases both output (quantity) and penetrating ability (quality).

Distance (SID)

Exposure is governed by the inverse square law of radiation. The farther away from the source, the less beam intensity. The exposure at a given distance from a point source is inversely proportional to the square of the distance.

Inverse Square Law Definition

States that beam intensity is inversely proportional to the square of the distance between the beam source and the image receptor (IR).

Inverse Square Law Formula

E1 / E2 = D2² / D1² — where E1 = initial exposure, D1 = initial distance, E2 = new exposure, D2 = new distance.

Effect of Distance on Exposure

Doubling the distance from the source will decrease exposure (beam intensity) to one-fourth (¼) of the original.